Differential phase-contrast imaging is used to enhance contrast of low absorbing specimen, compared to conventional amplitude contrast images, for example. In WO 2004/071298 A1, an apparatus is provided for generating phase-contrast X-ray image, which comprises, along an optical path, an incoherent X-ray source, a first beam splitter grating, a second beam recombiner grating, an optical analyzer grating and an image detector. In differential phase-contrast imaging, the grating G1, i.e. the so-called phase grating, is typically a pure phase grating with a pitch G1 that imposes a phase shift onto the phase front of the coherent X-ray radiation. After further propagation of the wave front to the analyzer grating G2 with a pitch in the order of G2=½G1, the intensity of the beam is modulated with a period equal to G2. Using an analyzer grating that has a modulation of its transmission with this pitch, the detector signal behind the analyzer grating has a periodicity of G2 and the phase of the signal can be used to derive position of the intensity maxima and thus the gradient of the phase front.